The 21-year-old has agreed a three-and-a-half-year deal but will immediately join Championship outfit, Middlesbrough, on loan until the end of the 2017/18 season.
Harrison returns to England after a stellar 2017 MLS campaign which enhanced his reputation as one of the most exciting young prospects in the United States.
Operating from a wide position, he found the back of the net 14 times in 59 appearances during his two seasons at the Yankee Stadium and his fine form earned him a call-up to Aidy Boothroyd’s England U21 squad for the Euro 2019 qualifiers against Scotland and Andorra in October.
Having moved to the United States as a 14-year-old, this will be the first time Harrison has played professionally in England and he spoke of his excitement ahead of the next stage of his burgeoning career.
“I can’t wait to get started over here,” he said.
“I’ve really enjoyed my time with New York City and I’d like to thank Patrick Vieira and his coaching staff for helping me to develop as a player.
“I feel I’ve really improved working under him and playing regularly in the MLS, but now I’m looking forward to testing myself in England.
“Middlesbrough are a good club, with Premier League experience and they’re in the mix for a return via the playoffs this season.
“Hopefully I can help them achieve that.”
After his move stateside, Harrison picked up a host of individual accolades as he progressed through the NCAA college system and in 2016, as the youngest eligible player in the MLS SuperDraft, he was picked first by Chicago Fire before New York City acquired him in exchange for allocation money and the fourth overall pick in the draft.
Blessed with pace and trickery, Harrison is an exciting winger who loves to commit the full-back and enjoyed an impressive debut season which saw him earn a Rookie of the Year nomination.
He continued to progress in the 2017 campaign, adding goals and assists to his game to reinforce his status as one of the best young players in the MLS.